/* K&R Exercise 4-3 p. 79 */

/* Given the basic framework, it's straightforward to extend the calculator.
 * Add the modulus (%) operator and provisions for negative numbers. */

#include <stdio.h>
#include <stdlib.h>     /* got atof() */

#define MAXOP 100       /* max size of operand or operator */
#define NUMBER '0'      /* signal that a number was found */

int getop(char []);
void push(double);
double pop(void);

/* reverse Polish calculator */
int main()
{
    int type;
    double op2;
    char s[MAXOP];

    while ((type = getop(s)) != EOF)
    {
        switch (type)
        {
            case NUMBER:
                push(atof(s));
                break;
            case '+':
                push(pop() + pop());
                break;
            case '*':
                push(pop() * pop());
                break;
            case '-':
                op2 = pop();
                push(pop() - op2);
                break;
            case '/':
                op2 = pop();
                if (op2 != 0.0)
                {
                    push(pop() / op2);
                }
                else
                {
                    printf("error: zero divisor\n");
                }
                break;
            case '%':
                op2 = pop();
                if (op2 != 0.0)
                {
                    push((double)((int)pop() % (int)op2));
                }
                else
                {
                    printf("error: zero divisor\n");
                }
                break;
            case '\n':
                printf("\t%.8g\n", pop());
                break;
            default:
                printf("error: Unknown command %s\n", s);
                break;
        }
    }

    return 0;
}

#define MAXVAL 100      /* maximum depth of val stack */

int sp = 0;             /* next free stack position */
double val[MAXVAL];     /* value stack */

/* push: push f onto value stack */
void push(double f)
{
    if (sp < MAXVAL)
    {
        val[sp++] = f;
    }
    else
    {
        printf("error: stack full, can't push %g\n", f);
    }
}

/* pop: pop and return top value from stack */
double pop(void)
{
    if (sp > 0)
    {
        return val[--sp];
    }
    else
    {
        printf("error: stack empty\n");
        return 0.0;
    }
}

#include <ctype.h>
int getch(void);
void ungetch(int);

/* getop: get next operator or numeric oeprand */
int getop(char s[])
{
    int i, c;

    while ((s[0] = c = getch()) == ' ' || c == '\t')
        ;

    s[1] = '\0';

    if (!isdigit(c) && c != '.' && c != '-')
    {
        return c;   /* not a number */
    }

    i = 0;

    /* If we found a '-' we need to make sure it is a minus sign and not a 
     * negative number. */
    if (c == '-')
    {
        while (isdigit(s[++i] = c = getch()))
            ;

        if (!isdigit(s[1]))
        {
            ungetch(c);
            return s[0];   /* not a number afterall */
        }
    }

    if (isdigit(c))     /* collect integer part */
    {
        while (isdigit(s[++i] = c = getch()))
            ;
    }

    if (c == '.')       /* collect fraction part */
    {
        while (isdigit(s[++i] = c = getch()))
            ;
    }

    s[i] = '\0';

    if (c != EOF)
    {
        ungetch(c);
    }

    return NUMBER;
}

#define BUFSIZE 100

char buf[BUFSIZE];      /* buffer for ungetch */
int bufp = 0;           /* next free position in buf */

int getch(void)         /* get a (possibly pushed back) character */
{
    return (bufp > 0) ? buf[--bufp] : getchar();
}

void ungetch(int c)     /* push character back on input */
{
    if (bufp >= BUFSIZE)
    {
        printf("ungetch: too many characters\n");
    }
    else
    {
        buf[bufp++] = c;
    }
}

